Heavier summer downpours with climate change

June 2014 - A recent research paper which appeared in Nature Climate Change has looked at how summer rainfall may change in future with a changing climate. Here, the lead author of the paper, Dr Lizzie Kendon from the Met Office, gives some additional insight into some aspects of the research.

This research aimed to understand how hourly rainfall may change in future with global warming. This involved running a very high resolution model, similar to the one we use for forecasting, to look at summer rainfall patterns over a 13-year period both now and at 2100.

Clearly potential conditions around 2100 in the UK are subject to variables such as the amount of human greenhouse gas emissions emitted, so we have to make some assumptions. We chose to use the IPCCRCP8.5 scenario - which is their highest end scenario which would see the most warming.

Why did we choose the highest IPCC (RCP8.5) scenario?

In this study we are trying to understand how hourly rainfall may change in the future with global warming. We chose the highest scenario in order to allow us to identify any signal of change above natural climate variability. The fact that we used the highest scenario, however, doesn't mean we cannot translate our findings to lower emission scenarios or to time periods earlier this century. What we have confidence in is the direction of the change - namely in an increase in heavy summer downpours in future. That direction of change is not expected to vary between high and low emissions scenarios. Further research in the future could help narrow down how much heavier rainfall is likely to be under different IPCC scenarios and give more detail overall, but - given how computer intensive the study was - this is likely to take some time.

Why does our study only consider the southern UK?

The high resolution climate model used in this study, needed to allow us to examine changes in short-duration intense storms, is very computationally expensive. Even running for just the southern half of the UK, it took the Met Office's supercomputer - one of the most powerful in the world - nine months to complete the simulations.

The southern UK was chosen for this initial study as convective storms, which are responsible for delivering intense summer downpours, are more common in that area. In addition we are interested in examining urban effects, and London is the largest urban area in the UK - so it made sense to include it in the study area. Similar climate change simulations for the northern half of the UK are currently being set up, and will be examined in a future study.

How robust are these results?

These results are based on one climate model and so we cannot assess modelling uncertainties. Although this model shows almost five times more events exceeding high thresholds indicative of serious flash flooding, we need to do more research before we can be confident of this figure. We have more confidence in the direction of the change - with increases in the intensity of heavy rain consistent with what we expect theoretically as the world warms. We need to wait for other centres to run similarly detailed simulations to see whether their results support these findings.